1. Field of the Invention
The present invention is directed to an image forming apparatus, such as a copier or printer, in which an electrophotographic method is employed for a contact-charging in which a charge member has a voltage applied thereto and is held in contact with a photosensitive body and a surface of the photosensitive body is charged. More particularly, the present invention is directed to a toner used in a developing device of the image forming apparatus.
2. Discussion of the Background
In a conventional image forming apparatus utilizing an electrophotographic method represented by a Carlson process, there is a non-contact-charging method in which the surface of the photosensitive body is uniformly charged. A high voltage is applied to a tungsten wire (30-100 .mu.m) called a corona wire. Corona discharge is performed between the corona wire and the photosensitive body. The surface of the photosensitive body is charged. As a result, air is ionized and a great amount of ozone and nitric oxide are generated.
Ozone and nitric oxide are harmful to the human body and aggravate deterioration of the photosensitive body as well as mechanical elements. While negative discharge is performed, ozone is generated in a very great amount. Recently, the photosensitive body became an organic photosensitive body for negative discharge. Environmental standards for gas exhausted from the image forming apparatus may also now be restricted, which causes serious problems.
As compared with the non-contact-charging method, there is a contact-charging method in which a charge member is held in contact with the photosensitive body and the surface of the photosensitive body is charged. In this operation a voltage applied to the charge member is low. A very small amount of ozone is then generated. These are advantages of the contact-charging method. An image forming apparatus provided with a roller-shaped contact charge member has been on the market.
However, in respect of uniformity of charge distribution, the contact-charging method is inferior to the non-contact-charging method with the corona wire.
Japanese Patent Laid-Open Publications No. 1249668/1988 teaches that the uniformity of charge distribution can be noticeably improved. This reference discloses that an AC voltage having a peak-to-peak voltage more than twice as high as a charge start voltage is superposed to the contact charge member in the event of application of a DC voltage.
However, there are the following problems in the above mentioned scheme. First, a size of a power source for applying the voltage increases. Also, a high frequency sound is generated by application of the AC voltage. Further, as the voltage is not efficiently used, an amount of ozone generation increases. Also, with application of the AC voltage, the material of the charge member is not apt to be restricted. There also arises a problem to be solved in that toner is easily fused to the charge member by a vibration electric field.
The uniformity of charge distribution performed with application of only the DC voltage without the AC voltage is studied. However, the material of the charge member is apt to be restricted. Even though material of good uniformity could be found, toner and paper dust attach to the charge member. Because the charge member is constantly held in contact with the photosensitive body, variation of an electric resistance value occurs partially, and this results in non-uniformity of charge distribution.
It has been suggested that the charge member be provided with a cleaning member so as to clean a surface of the charge member. Japanese Patent Laid-Open Publication No. 101768/1991 teaches that a sponge material (polyurethane foam, polyethylene foam) is held in contact with the surface of the charge member and cleans the surface.
However, the surface of the charge member is made of resin or rubber in general. As a result, it is difficult to remove toner attached to the surface of the charge roller. If the cleaning member is held in contact with the surface of the charge member with high pressure, toner can be removed, but the surface of the charge member is easily scratched. If the cleaning member is held in contact with the surface of the charge member with low pressure so as not to scratch the surface of the charge member, toner remains in the surface as an undesirable lateral strip. There then arises a problem to be solved that adjustment for contact pressure to the surface of the charge member is difficult.